1. Field of the Invention
This invention relates to thermally responsive semiconductor actuators, and in particular to semiconductor microactuators having deformable members comprised of a bimetallic material where the thermally insulative hinge between the frame and the diaphragm of the actuator is of a uniform thickness and the hinge structure is comprised of two hinges which are parallel for at least part of their lengths.
2. Description of Related and Commonly Invented Art
It is well known to use silicon devices as transducers for converting physical quantities such as force, pressure, temperature, and acceleration into electrical signals which may be provided to electrical processing circuitry. In addition, silicon devices are sometimes used as actuators or transducers, for instance for operating valves.
See, for instance, commonly invented U.S. Pat. No. 5,069,419 issued Dec. 3, 1991, entitled "Semiconductor Microactuator".
FIGS. 1-4 herein are identical to the similarly numbered figures in U.S. Pat. No. 5,069,419. As shown in FIG. 1, the semiconductor microactuator 10 has a silicon semiconductor substrate 12 formed from a crystalline silicon die fabricated from a monocrystalline silicon wafer and having a thickness of 300 micrometers. Suspension means 14 is connected to substrate 12. A movable element 16 is connected to the suspension means 14 to be displaced solely translationally, or in other words irrotationally, with respect to the semiconductor substrate 12.
Suspension means 14, as seen in FIGS. 1, 2, and 4, includes a hinge 18 comprised of a layer of thermally grown silicon oxide. Hinge 18 is formed integrally with a thinner layer of thermally grown silicon oxide 20 extending over the epitaxial silicon substrate 12. The hinge portion of the thermally grown oxide layer 20 has a thickness of about 3 micrometers (30,000 .ANG.). The other portions of thermally grown oxide layer 20 have a thickness of about 2000 .ANG.. The oxide layer 20 is primarily used for passivation and for electrical insulation. The hinge 18 is circular and in part, defines a circular diaphragm 22, which is connected to it about its periphery. The diaphragm 22 includes a silicon body portion 24 having a boss 26 formed integrally therewith. A thinner diaphragm portion 28 has a heater 30 formed therein comprising a first heater ring 32 and a second heater ring 34. The first and second heater rings 32 and 34 are concentric with the circular diaphragm 22 and are comprised of diffused regions of the monocrystalline silicon, as seen in FIGS. 2 and 3. The silicon oxide layer 20 covers the diffused regions 32 and 34 to insulate them electrically from other portions of the semiconductor microactuator 10. The circular metal ring 36 comprised of electron beam or sputter deposited metal covers a portion of the oxide layer over the heater rings 32 and 34. The heater rings 32 and 34 are connected via metal leads 38 and 40 to diffused current supply regions 42 and 44. The lead 38 is connected to an aluminum bonding pad 46 on the substrate 12. Suitable leads may be wire bonded to the bonding pads 46 and 48 to supply electric current to the bonding pads through the leads 38 and 40 and to the diffused regions 32 and 34.
When the heater rings 32 and 34 receive electric current they heat, causing the thinner diaphragm portions 28 to heat and expand at a thermal expansion rate governed by the thermal expansion coefficient of monocrystalline silicon. The metal ring 36 lying above the diffused regions 32 and 34 also expands, but at a higher rate, due to its greater thermal expansion coefficient, causing the thinner portions 28 to bow and displacing the diaphragm 22. Thus, by controlling the amount of electric current fed to the diffused regions 32 and 34 the amount of displacement of the diaphragm 16 can also be controlled. In addition, since the displacing force is being supplied throughout the entire periphery of the thinner diaphragm region 28, it may be appreciated that relatively high force may be supplied to the movable member or boss portion 26 of the diaphragm so that useful work can be done, for instance, operating a valve.
U.S. Pat. No. 5,069,419 discloses other embodiments of this structure and is incorporated herein by reference. Specifically this patent shows, for instance, use of the above-described microactuator in conjunction with a base substrate including a valve orifice and seat which is contacted by the boss 26, thus closing the valve when the boss is in contact with the valve seat and opening the valve when the boss 26 is not in contact with the valve seat.
The structure of the hinge 18 shown in FIG. 4, however, is deficient in several respects. Hinge 18 of FIG. 4 typically is formed by providing an etch stop in the silicon diaphragm 12 except in the area where the hinge 18 is located. Then when the silicon diaphragm 12 is etched from its backside, a short overetching will remove the silicon beneath the hinge 18. However, in practice it is difficult or inconvenient to provide this etch stop. A heavily doped boron etch stop may be used; however, the heavy doping affects the stress in the silicon diaphragm which makes control of diaphragm deflection difficult and hence reduces the utility of the device. It is possible to use electrochemical etch stops in the silicon, but the complexity of this process is high and there are attendant problems in making a narrow width hinge due to the lateral etch behavior which is a well known property of electrochemical etch processes.
Additionally, a significant function of the hinge 18 is to provide the mechanical boundary condition of the bimetallic diaphragm 12 such that the deflection of the diaphragm 12 is approximately that of a simply supported diaphragm. The difference between clamped and simply supported boundary conditions for bimetallic diaphragms and beams is well known to provide substantially different deflection characteristics of the diaphragm 12, including changing the direction and magnitude of the deflection with increasing temperature of the diaphragm 12. The hinge structure of FIG. 4 is not optimum for providing both this hinge mechanical action and isolating compressive stresses generated by the heating of the bimetallic diaphragm.
Therefore, there is a need for a hinge which is both easier to fabricate and provides more uniform mechanical performance than does the structure of FIG. 4.